Implementation of a robust RELP speech coder

This paper presents a RELP coder based on base-band transmission and high-frequency regeneration in the frequency domain in two versions: 4.8 kb/s and 9.6 kb/s. The principle of both versions is illustrated in the detailed block diagram in Fig.1. Some details of a fixed-point implementation will be described. The 4.8 kb/s version has been implemented on a real-time processor. Since the algorithm does not distinguish between voiced and unvoiced speech, the number of calculations per frame is nearly constant. Thus a 4% time reserve is sufficient to insure real-time operation for the 4.8 kb/s version. The robustness of the RELP coder in the presence of background noise is explained. Except for narrow-band high-frequency background noise, the RELP coder transmits this noise as well as speech without any significant distortion. The influence of transmission errors is described. Simulations with a mobile radio channel model showed that the quality of the speech transmitted ´digitally´ by RELP and a digital modem is definitely superior to that transmitted by simulated analog FM.